The present invention relates generally to the field of security documents and more particularly to the printing of images and security documents incorporating at least one metallic ink. By combining a metallic ink and transparent inks, one may create printed images behaving dynamically: an image viewed under specular reflection may be considerably different from the same image viewed under non-specular reflection. Patterns which are either dark or hidden become highlighted under specular reflection, yielding interesting visual effects. Metallic inks allow therefore to create visually appealing dynamically changing color images. In addition, the inversion effect provided by metallic inks allows to enhance microstructures present in the image or to reveal hidden patterns. This is especially useful for the creation of security documents. This is of high importance, since counterfeiting of documents such as banknotes is becoming now more than ever a serious problem, due to the availability of high-quality and low-priced color photocopiers and desk-top publishing systems. The same is also true for other valuable products such as CDs, DVDs, software packages, medical drugs, luxury goods, etc., that are often marketed in easy to falsify packages. The present invention aims at providing a novel security element and authentication means offering enhanced security for banknotes, checks, credit cards, identity cards, travel documents, industrial packages or any other valuable articles, thus making them much more difficult to counterfeit. In addition the present invention discloses methods for separating an input color image into a metallic ink and transparent inks. This may be used also to create color images having a metallic appearance at specular reflection angles and therefore enlarge the design space of artists. For design purposes and for increasing the security of documents and valuable goods against counterfeiting, we also disclose a method for integrating metallic elements into regions of an image. Thanks to the integrated metallic elements, the visibility of microstructure patterns embedded into a color image can be enhanced at specular angles. Moreover, support can be offered for designs and security documents making use of the inversion effect, i.e. transitions between darkness at non-specular reflection angles and highlights at specular reflection angles. Furthermore, ghost patterns may be hidden at non-specular angles and become apparent at specular angles.
A prior art method called “Metallic Color Printing Process” is disclosed in U.S. Pat. No. 5,370,976 and describes a color separation method for printing images incorporating metallic inks such as gold or silver in addition to standard cyan, magenta, yellow, and black inks. This as well as other prior art approaches for printing with metallic inks (see section background in U.S. Pat. No. 5,370,976) rely on a simple undercolour removal technique. A metallic ink has, under non-specular viewing conditions, an assigned c′m′y′ (cyan, magenta, yellow) equivalent, which is subtracted from the original cmy (cyan, magenta, yellow) values in order to yield the amounts of csmsys to be printed on top of the metallic ink. However, since the combination of ink layers is not really subtractive, this method cannot ensure a high reproduction fidelity. Furthermore, that prior art method is not able to predict the colorimetric values (e.g. the CIE-XYZ tri-stimulus values) neither of the patch with the original ink coverage cmy (cyan, magenta, blue) values, nor of patch with the coverage values csmsys of the inks to be printed by superposing transparent inks and the metallic ink.
To ensure a high reproduction fidelity, one needs to rely on a scientific model predicting the spectra (or the calorimetric values) of printed ink patches. Establishing a spectral prediction model which is valid for printing both with transparent inks and with a combination of a metallic ink and transparent inks represents a considerable challenge. Such a prediction model is disclosed in detail in the present invention and used for predicting the reflection spectrum (and therefore also the colorimetric values) of patches printed either with or without a metallic ink layer.
In respect to security documents, non-standard inks such as metallic or iridescent inks are already used for protecting security documents and valuable articles such as banknotes, tickets, etc. See for example the Swiss banknotes. Metallic inks however are generally printed either alone or on top of an existing image. Since they are not really embedded into an image, they can be reproduced without much effort. The present invention discloses how to embed metallic ink patterns into printed color images.
One prior art method for the creation of patterns visible at specular reflection angles consists in overlaying on top of an image a transparent varnish. Since the varnish is transparent at non-specular reflection angles and reflects light at specular reflection angles, patterns become visible at specular reflection angles. However, the reflected light has the same color as the incident light, i.e. in the general case white. In contrast to a varnish, the superposition of a metallic ink and transparent inks yields a colored reflected light, whose color depends on the color and the amounts of transparent inks superposed on top of the metallic ink.
Patent application GB2375322, Security Device, inventors J. L. Thick, A. Nutton and R. Bratchley (also published as PTO WO0166360 and AU376320, priority date 10 Apr. 2000) teaches a method for securing documents by printing an opaque, white or lightly colored diffusely reflective ink on top of a specular reflective background, e.g. a solid metallic ink. By tilting the document, the specular reflective background is either dark or highlight, making the diffusely reflective ink appear by contrast as respectively either highlight (white) or dark. In contrast to that method, the present invention discloses how to synthesize and print a full color image comprising a metallic ink and several standard inks, said standard inks being transparent, i.e. not diffusely reflecting or diffusely reflecting to a negligible extent.